Every year, pests detrimental to agriculture, forestry, and public health cause losses in the millions of dollars. Various strategies have been used to control such pests.
One strategy is the use of chemical pesticides with a broad range or spectrum of activity. However, there are a number of disadvantages with using chemical pesticides. Specifically, because of their broad spectrum of activity, these pesticides may destroy non-target organisms such as beneficial insects and parasites of destructive pests. Additionally, chemical pesticides are frequently toxic to animals and humans. Furthermore, targeted pests frequently develop resistance when repeatedly exposed to such substances.
Another strategy involves the use of biopesticides to control insect, fungal and weed infestations. Biopesticides are naturally occurring pathogens and/or the substances produced bythese pathogens. The advantage of using biopesticides is that they are generally less harmful to non-target organisms and the environment as a whole compared to chemical pesticides.
2.1. Bacillus thuringiensis
The most widely used biopesticide is Bacillus thuringiensis. Bacillus thuringiensis is a motile, rod-shaped, gram-positive bacterium that is widely distributed in nature, especially in soil and insect-rich environments. During sporulation, Bacillus thuringiensis produces a parasporal crystal inclusion(s) which is insecticidal upon ingestion to susceptible insect larvae of the orders Lepidoptera, Diptera, and Coleoptera. The inclusions may vary in shape, number, and composition. They are comprised of one or more proteins called delta-endotoxins, which may range in size from 27-140 kDa. The insecticidal delta-endotoxins are generally converted by proteases in the larval gut into smaller (truncated) toxic polypeptides, causing midgut destruction, and ultimately, death of the insect (Hofte and Whiteley, 1989, Microbiological Reviews 53:242-255).
There are several Bacillus thuringiensis strains that are widely used as biopesticides in the forestry, agricultural, and public health areas. Bacillus thuringiensis subsp. kurstaki and Bacillus thuringiensis subsp. aizawai produce delta-endotoxins specific for Lepidoptera. A delta-endotoxin specific for Coleoptera is produced by Bacillus thuringiensis subsp. tenebrionis (Krieg et al., 1988, U.S. Pat. No. 4,766,203). Furthermore, Bacillus thuringiensis subsp. israelensis produces delta-endotoxins specific for Diptera (Goldberg, 1979, U.S. Pat. No. 4,166,112).
Other Bacillus thuringiensis strains specific for dipteran pests have also been described. A Bacillus thuringiensis isolate has been disclosed which is toxic to Diptera and Lepidoptera (Hodgman et al., 1993, FEMS Microbiology Letters 114:17-22). SDS polyacrylamide gel electrophoresis of the purified crystal delta-endotoxin from this isolate revealed three protein species which are related to CryIA(b), CryIB, and CryIIA toxins. There has also been disclosed a Bacillus thuringiensis isolate which produces a dipteran-active crystal comprised of proteins with molecular weights of 140, 122, 76, 72, and 38 kDa (Payne, 1994, U.S. Pat. No. 5,275,815). EPO 480,762 discloses five B.t. strains which are each active against dipteran pests; each also have a unique crystal delta-endotoxin pattern.
Several Bacillus thuringiensis strains have been described which have pesticidal activity against pests other then Lepidoptera, Coleoptera, and Diptera. Five Bacillus thuringiensis strains have been disclosed which produce delta-endotoxins that are toxic against nematodes (Edwards, Payne, and Soares, 1988, Eur. Pat. Appl. No. 0 303 426 B1). There has also been disclosed a Bacillus thuringiensis strain, PS81F, which can be used to treat humans and animals hosting parasitic protozoans (Thompson and Gaertner, 1991, Eur. Pat. Appl. No. 0 461 799 A2). Several Bacillus thuringiensis isolates have also been disclosed with activity against acaride pests. These isolates produce crystals comprised of proteins with molecular weights in the (wide) range of 35 kDa to 155 kDa (Payne, Cannon, and Bagley, 1992, PCT Application No. WO 92/19106). There have also been disclosed Bacillus thuringiensis strains with activity against pests of the order Hymenoptera (Payne, Kennedy, Randall, Meier, and Uick, 1992, Eur. Pat. Appl. No. 0 516 306 A2); with activity against pests of the order Hemiptera (Payne and Cannon, 1993, U.S. Pat. No. 5,262,159); with activity against fluke pests (Hickle, Sick, Schwab, Narva, and Payne, 1993, U.S. Pat. No. 5,262,399; and with activity against pests of the order Phthiraptera (Payne and Hickle, 1993, U.S. Pat. No. 5,273,746). Furthermore, another strain of Bacillus thuringiensis subsp. kurstaki, WB3S-16, isolated from Australian sheep wool clippings, has been disclosed that is toxic to the biting louse Damalinia ovis, a Phthiraptera pest (Drummond, Miller, and Pinnock, 1992, J. Invert. Path. 60:102-103).
The delta-endotoxins are encoded by cry (crystal protein) genes which are generally located on plasmids. The cry genes have been divided into six classes and several subclasses based on relative amino acid homology and pesticidal specificity. The major classes are Lepidoptera-specific (cryI); Lepidoptera-and Diptera-specific (cryII); Coleoptera-specific (cryIII); Diptera-specific (cryIV) (Hofte and Whiteley, 1989, Microbiological Reviews 53:242-255); Coleoptera- and Lepidoptera-specific (referred to as cryV genes by Tailor et al., 1992, Molecular Microbiology 6:1211-1217); and Nematode-specific (referred to as cryV and cryVI genes by Feitelson et al., 1992, Bio/Technology 10:271-275).
Delta-endotoxins have been produced by recombinant DNA methods. The delta-endotoxins produced by recombinant DNA methods may or may not be in crystal form.
Some strains of Bacillus thuringiensis have been shown to produce a heat-stable pesticidal adenine-nucleotide analog, known as .beta.-exotoxin type I or thuringiensin, which is pesticidal alone (Sebesta et al., in H. D. Burges (ed.), Microbial Control of Pests and Plant Diseases, Academic Press, New York, 1980, pp. 249-281). .beta.-exotoxin type I has been found in the supernatant of some Bacillus thuringiensis cultures. It has a molecular weight of 701 and is comprised of adenosine, glucose, and allaric acid (Farkas et al., 1977, Coll. Czechosslovak Chem. Comm. 42:909-929; Luthy et al., in Kurstak (ed.), Microbial and Viral Pesticides, Marcel Dekker, New York, 1982, pp. 35-72). Its host range includes, but is not limited to, Musca domestics, Mamestra configurata Walker, Tetranychus urticae, Drosophila melanogaster, and Tetranychus cinnabarinus. The toxicity of .beta.-exotoxin type I is thought to be due to inhibition of DNA-directed RNA polymerase by competition with ATP. It has been shown that S-exotoxin type I is encoded by a cry plasmid in five Bacillus thuringiensis strains (Levinson et al., 1990, J. Bacteriol. 172:3172-3179). .beta.-exotoxin type I was found to be produced by Bacillus thuringiensis subsp. thuringiensis serotype 1, Bacillus thuringiensis subsp. tolworthi serotype 9, and Bacillus thuringiensis subsp. darmstadiensis serotype 10.
Another .beta.-exotoxin classified as .beta.-exotoxin type II has been described (Levinson et al., 1990, J. Bacteriol. 172:3172-3179). .beta.-exotoxin type II was found to be produced by Bacillus thuringiensis subsp. morrisoni serotype 8ab and is active against Leptinotarsa decemlineata. The structure of .beta.-exotoxin type II is not completely known, but is significantly different from that of .beta.-exotoxin type I in that a pseudouridine moiety is in the place of adenine in which attachment to the ribose ring is at a position that would otherwise be occupied by a proton (Levinson, in Hickle and Finch (eds.), Analytical Chemistry of Bacillus thuringiensis, ACS Symposium Series, Washington, D.C., 1990, pp. 114-136). Furthermore, there is only one signal in the proton NMR spectrum corresponding to the nucleoside base (at 7.95 ppm), and does not have a ribose-type anomeric protein signal (5.78 ppm).
Other water soluble substances that have been isolated from Bacillus thuringiensis include alpha-exotoxin which is toxic against the larvae of Musca domestica (Luthy, 1980, FEMS Microbial. Lett. 8:1-7); gamma-exotoxins, which are various enzymes including lecithinases, chitinases, and proteases, the toxic effects of which are expressed only in combination with beta-exotoxin or delta-endotoxin (Forsberg et al., 1976, Bacillus thuringiensis: Its Effects on Environmental Quality, National Research Council of Canada, NRC Associate Committee on Scientific Criteria for Environmental Quality, Subcomittees on Pesticides and Related Compounds and Biological Phenomena); sigma exotoxin which has a structure similar to beta-exotoxin, and is also active against Leptinotarsa decemlineata (Argauer et al., 1991, J. Entomol. Sci. 26:206-213); and anhydrothuringiensin (Prystas et al., 1975, Coll. Czechosslovak Chem. Comm. 40:1775).
2.2. Zwittermicin
A substance has been isolated from Bacillus cereus which inhibits the growth of the plant pathogen Phytophthora medicaginis and reduces the infection of alfalfa (see, for example, U.S. Pat. Nos. 4,877,738 and 4,878,936). No other activity was disclosed. The following structure has been elucidated for zwittermicin A (He et al., Tet. Lett. 35:2499-2502): ##STR1##